The invention described herein relates to mobile "jack-up" drilling and/or service units, and more particularly to a system for protecting the unit's jacking system from additional loading beyond its nominal jacking capacity.
A "jack-up" unit as used herein means any self-elevating platform used for drilling, production, well service, construction or other offshore/inshore purpose. A typical unit consists of a platform structure with attached legs. Said legs are elevated above the platform for movement of the unit from one location to another. Upon reaching the desired location the legs are lowered and upon touching bottom the jacking system, having adequate lifting capacity, raises the platform above the water surface providing a stable working platform. Upon completion of work at the site, the platform is lowered to the water, the legs retracted to the elevated position above the platform, and the unit is moved to the next location.
The present invention was originally conceived as a preload mechanism utilized to limit the maximum loadings on a typical rack and pinion jacking system. However, the present invention is applicable as a means to limit the maximum loadings on any types of jacking systems. In addition, the source of the increased load requirement is not limited to preload application, but may be due to increased variable load while operating, wind and/or wave loadings, and long term soil settlement. The following discussion is directed toward but not limited to the preferred embodiment of the present invention.
The invention described herein, is intended to apply to any jack-up unit with one or more legs and equipped with a jacking system. A jacking system commonly used but not exclusive is the rack and pinion type; however, the system is applicable to any progressively engaging system, be it of rack and pinion or other form.
A "preload" sequence is utilized to reduce the possibility of damage to the unit's legs, platform structure, and/or jacking system as a result of unanticipated soil failure below one or more of the footings. The preload sequence is initiated immediately after the platform is elevated several feet above the crest elevation of any minor wave action on the water surface. As the elevating process is stopped, the axial loadings on the legs are the nominal jacking loads and are transferred from the platform structure through the jacking system to the leg structure and ultimately to the soil below the leg footings.
In anticipation of large wave crest elevations that may occur as a result of storm action while operating, the unit needs to be elevated some twenty-five (25) to fifty (50) feet above the still water elevation before work is commenced. However, the bearing capacity of the soil under the footings is often uncertain, and if a soil failure were to occur with the unit elevated to its large operating "air gap" above the water surface, a catastrophic amount of damage could occur to the unit's platform, leg, and jack structure. Preloading the legs to an axial load value that exceeds any anticipated increase in leg load at a future time while the unit is still near the water surface reduces the possibility of damage and/or minimizes the damage that may occur if soil failure is experienced during the preload sequence. The benefit derived by experiencing any soil failure with a small clearance between platform and water surface can be attributed to the ability of a buoyant hull platform to stop leg penetration into the soil as the hull reenters the water and regains buoyancy before the unit experiences a drastic inclination toward the direction of the leg(s) footing(s) experiencing soil failure.
The additional leg loadings that may occur at a future time are caused by increased variable load desired on the unit during work progress and/or wind and wave overturning moments due to storm conditions that may occur during the work process. A careful review of anticipated increases in deck load due to work requirement as well as an engineering evaluation of increased leg load due to extreme storm overturning moments for the operational area permits the extreme leg loading that will be experienced at any one location to be estimated. Preloading sequences that employ an adequate factor of safety above the maximum estimated leg loadings greatly reduce the possibility of catastrophic soil failure after the unit has been elevated to its normal operating air gap.
Units equipped with independent legs normally require the addition of load to the platform when the platform is elevated to its small preload air gap to effect adequate leg load increases. This is commonly accomplished by pumping readily available water into specially provided ballast tanks in the hull platform. Units with four (4) or more legs may be preloaded by this ballasting sequence or by raising one (1) or more legs in a prescribed sequence to increase the loading on the legs maintaining soil contact without the necessity of adding additional load to the platform. Regardless of which method is used to preload the legs, existing jack-up units require an additional capacity of the active jacking system above that required to elevate the platform in order to effect the required preload value for operation.
To overcome the recognized problems of additional load and soil failure, the invention described herein utilizes a portable large diameter gear or rack section with associated positioning and load transfer mechanisms to protect the active jacking system from the additional leg load applied to achieve required preload value. The preload mechanism system is engaged prior to the ballast transfer on units with three or fewer legs, or is engaged on diagonally opposite legs of a four (4) or more legged system before the legs to be retracted are moved. By this action the maximum loading that is placed on the active jacking system never exceeds the nominal jacking capacity required to elevate the unit, thereby allowing a substantial savings in cost and weight on the jacking units and associated rack attached to the unit's legs. No other prior art system offers this advantage.